Liquid crystal display panel and method for manufacturing the same

ABSTRACT

An OCB mode liquid crystal display panel comprises a pair of electrode substrates, a liquid crystal layer which is held between the pair of electrode substrates and has liquid crystal molecules whose alignment state is controlled within a display area by a drive voltage applied from the electrode substrates, and a pair of alignment films which are disposed on the electrode substrates in contact with the liquid crystal layer and whose rubbing directions are set parallel to each other. The concentration of the impurity ions contained in that part of the liquid crystal layer which is located inside the display area is lower than the concentration of the impurity ions contained in that part of the liquid crystal layer which is located outside the display area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2004-159659, filed May 28, 2004,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display panel havingan optically compensated bend (OCB) mode and a method for manufacturingthe same.

2. Description of the Related Art

Liquid crystal display panels have widely been used as image displaysfor computers, automobile navigation systems, monitors and TVs. TN andSTN modes, using nematic liquid crystals are popular as liquid crystaldisplay modes for the liquid crystal display panels. Other liquidcrystal display modes using ferroelectric liquid crystals and the likeare also known and provide a higher response speed and a wider viewingangle. These display modes, however, require further improvement inimpact resistance and thermal properties. In contrast, as an opticallycompensated liquid crystal display mode that provides a higher responsespeed and a wider viewing angle, there is an OCB mode in which liquidcrystal molecules are aligned in parallel without being twisted. ThisOCB mode is focused on use in video equipment and active developmentthereof has been carried out.

In general, an active matrix liquid crystal display panel has astructure in which a liquid crystal layer is held between an arraysubstrate and a counter substrate. The array substrate includes aplurality of pixel electrodes arrayed substantially in a matrix, aplurality of scanning electrodes disposed along rows of the pixelelectrode, a plurality of data signal electrodes disposed along columnsof the pixel electrodes, and a plurality of switching elements disposednear intersections between the scanning electrodes and the data signalelectrodes, for example. Each of the switching elements is formed, forexample, of a thin-film transistor (TFT), and turns on to apply thepotential of one data signal electrode to one pixel electrode when onescanning electrode is driven. On the counter substrate, a counterelectrode is disposed to face the plurality of pixel electrodes arrayedon the array substrate. A pair of pixel and counter electrodes forms onepixel together with a pixel area of the liquid crystal layer, and thealignment state of liquid crystal molecules within the pixel area iscontrolled by the electric field corresponding to the drive voltage heldbetween the pixel electrode and the counter electrode.

In an OCB mode liquid crystal display panel, the liquid crystalmolecules are in a splay alignment before supply of power. This splayalignment is a state where the liquid crystal molecules are laid down,and obtained by alignment films which are disposed on the pixelelectrode and the counter electrode and rubbed in parallel with eachother. A display operation of the liquid crystal display panel beginsafter an initialization process for applying a transfer voltage to theliquid crystal layer upon supply of power to transfer the alignmentstate of the liquid crystal molecules from the splay alignment to a bendalignment by a relatively strong electric field corresponding to thetransfer voltage. The alignment state of the liquid crystal molecules ismaintained in the bend alignment during the display operation, so as toattain the higher response speed and wider viewing angle peculiar to theOCB mode.

In addition, the alignment state of the liquid crystal molecules isinverse-transferred from the bend alignment to the splay alignment whena long period has elapsed in a no-voltage-applied state or avoltage-applied state where a voltage is applied but this voltage isbelow a level at which the energy of the splay alignment and the energyof the bend alignment are balanced. In the OCB mode liquid crystaldisplay panel, black insertion driving is employed as a driving systemfor preventing the inverse transfer. In black insertion driving, aninverse-transfer preventing voltage and a display voltage correspondingto a video signal are alternately applied to the liquid crystal layer asthe drive voltage at every frame cycle to maintain the bend alignment.Since the OCB mode liquid crystal display panel is a display panel of anormally white mode, the inverse-transfer preventing voltage correspondsto a voltage for a black display. Thus, this driving scheme is calledblack insertion driving. Further, the ratio of a period of applying theinverse-transfer preventing voltage to the total period of applying thedisplay voltage and the inverse-transfer preventing voltage is calledblack insertion ratio.

In manufacture of the liquid crystal display panel, as shown in FIG. 5A,after an array substrate 1 and a counter substrate 2 are individuallyformed, rubbing treatment is applied to the alignment films on the arraysubstrate 1 and the counter substrate 2. Thereafter, the array substrate1 and the counter substrate 2 are bonded by use of a sealing resin layer3. The sealing resin layer 3 is applied to surround a liquid crystalfilling space and form an open part left as an inlet 4. Driver circuitelements are disposed along a first side of the array substrate 1, andthe inlet 4 is disposed near a second side of the array substrate 1 thatis opposed to the first side. The rubbing treatment for each alignmentfilm is carried out in the same direction from the second side to thefirst side so as to avoid electrostatic destruction of the drivercircuit elements. In FIG. 5A, RD1 denotes the rubbing direction of thealignment film on the array substrate 1, and RD2 denotes the rubbingdirection of the alignment film on the counter substrate 2. A liquidcrystal material is applied from the inlet 4 to the liquid crystalfilling space as the liquid crystal layer LQ, and the inlet 4 is sealedwith a sealing member 5.

It is inevitable that impurity ions get into the liquid crystal layer LQduring the above-mentioned manufacturing process. Specifically, thesealing member 5 is the main impurity ion source that supplies asignificant quantity of impurity ions to the liquid crystal layer LQ.Such impurity ions decrease the insulation resistance of the liquidcrystal. Thus, the display characteristics are impaired due to decreasedvoltage retention. Further, upon application of the drive voltage, theimpurity ions are moved in the liquid crystal layer LQ. When theimpurity ions are distributed unevenly, an image fault such asnon-uniformity in display occurs. For example, Jpn. Pat. Appln. KOKAIPublication No. 9-54325 discloses a technique of providing an ion trapelectrode in addition to electrodes arranged in one direction on asubstrate and sequentially applying a high potential pulse to theelectrodes to prevent uneven distribution of the impurity ions. However,it is difficult to use this technique as a solution to the problemoccurring in the OCB mode liquid crystal display panel.

In the OCB mode liquid crystal display panel, liquid crystal moleculeswithin the liquid crystal layer LQ are set in a bend alignment toperform a display operation. The orientation angle of each liquidcrystal molecule significantly changes between a white display state inwhich a small voltage is applied to the liquid crystal layer LQ and ablack display state in which a large voltage is applied to the liquidcrystal layer LQ. The change in the orientation angle causes drifting ofthe liquid crystal molecules. Thus, a flow of the liquid crystalmolecules in the drift direction occurs in the liquid crystal layer LQ.The direction of this flow coincides with the rubbing directions of thealignment films for aligning the liquid crystal molecules in the OCBmode liquid crystal display panel. When impurity ions are moved by theflow in the rubbing directions RD1 and RD2 shown in FIG. 5A, theimpurity ions DF having high concentration in the vicinity of the inlet4 are diffused in the liquid crystal layer LQ as shown in FIG. 5B.Further, non-uniformity in display occurs because the electric chargeretention is locally decreased as a result of continuous black insertiondriving. This can be confirmed, for example, by an operation ofcontinuously displaying an image of a test pattern shown in FIG. 5C inthe form of black insertion driving and then displaying an image of awhole black pattern. In this case, the later image is not displayedentirely in black, and gray stripes shown in FIG. 5D are observed asburn-in portions. As shown in FIG. 5C, the impurity ions drift from thewhite display regions and concentrated in the black display regions. Thegray stripes are created in portions where the electric charge orapplied voltage retention is locally decreased by the concentratedimpurity ions.

It is difficult to solve the burn-in problem occurring in the OCB modeliquid crystal display panel by applying a high potential pulse in thesame manner as the above-mentioned technique.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an OCB mode liquidcrystal display panel that can suppress non-uniformity in display causedby impurity ions when black insertion driving is carried out in adisplay operation, and a method for manufacturing the OCB mode liquidcrystal display panel.

According to the invention, there is provided a method for manufacturingan OCB mode liquid crystal display panel, which comprises steps of:forming a pair of electrode substrates, a liquid crystal layer which isheld between the pair of electrode substrates and has liquid crystalmolecules whose alignment state is controlled within a display area by adrive voltage applied from the electrode substrates, and a pair ofalignment films which are disposed on the electrode substrates incontact with the liquid crystal layer and whose rubbing directions areset parallel to each other, as one display panel; and performing anaging process for obtaining a flow of the liquid crystal molecules inthe rubbing directions of the alignment films by applying to the liquidcrystal layer a drive voltage that periodically changes in a blackinsertion ratio determined independently from that of black insertiondriving for preventing the alignment state of the liquid crystalmolecules from being inverse-transferred from a bend alignment to asplay alignment, the aging process being continued for a predeterminedperiod required for moving impurity ions contained in the liquid crystallayer outside the display area.

According to the invention, there is provided an OCB mode liquid crystaldisplay panel, which comprises: a pair of electrode substrates; a liquidcrystal layer which is held between the pair of electrode substrates andhas liquid crystal molecules whose alignment state is controlled withina display area by a drive voltage applied from the electrode substrates;and a pair of alignment films which are disposed on the electrodesubstrates in contact with the liquid crystal layer and whose rubbingdirections are set parallel to each other; wherein the concentration ofthe impurity ions contained in that part of the liquid crystal layerwhich is located inside the display area is lower than the concentrationof the impurity ions contained in that part of the liquid crystal layerwhich is located outside the display area.

With the OCB mode liquid crystal display panel and the method formanufacturing this panel, the concentration of the impurity ionscontained in that part of the liquid crystal layer which is locatedinside the display area is lower than the concentration of the impurityions contained in that part of the liquid crystal layer which is locatedoutside the display area, as a result of the aging process performedduring the manufacture. Thus, it is possible to reduce the quantity ofimpurity ions locally concentrated in the display area during the blackinsertion driving for a display operation, thereby suppressingnon-uniformity in display caused by the concentrated impurity ions.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an embodiment of the invention,and together with the general description given above and the detaileddescription of the embodiment given below, serve to explain theprinciples of the invention.

FIG. 1 is a diagram showing the plane structure of an OCB mode liquidcrystal display panel according to one embodiment of the invention;

FIG. 2 is a diagram showing the circuit configuration of the liquidcrystal display panel shown in FIG. 1;

FIGS. 3A to 3C are diagrams showing the cross sectional structure andthe liquid crystal alignment of the OCB mode liquid crystal displaypanel shown in FIG. 1;

FIG. 4 is a diagram showing rubbing treatment applied to each alignmentfilm shown in FIGS. 3A to 3C; and

FIGS. 5A to 5D are plan views explaining a problem that occurs in an OCBmode liquid crystal display panel.

DETAILED DESCRIPTION OF THE INVENTION

An OCB mode liquid crystal display panel LCD according to one embodimentof the present invention will be described with reference to theaccompanying drawings.

FIG. 1 shows the plane structure of the liquid crystal display panelLCD, FIG. 2 schematically shows the circuit configuration of the liquidcrystal display panel LCD, FIGS. 3A to 3C shows the cross sectionalstructure of the liquid crystal display panel LCD. As shown in FIG. 1,the liquid crystal display panel LCD includes an array substrate 1, acounter substrate 2, and a liquid crystal layer LQ held between thearray substrate 1 and the counter substrate 2. The array substrate 1 andcounter substrate 2 are a pair of electrode substrates, and bonded byuse of a sealing resin layer 3, which is applied to surround a liquidcrystal filling space between the substrates 1 and 2 and form an openpart left as an inlet 4. The liquid crystal layer LQ is obtained byfilling the liquid crystal filling space, for example, with a nematicliquid crystal material from the inlet 4, and sealing the inlet 4 with asealing member 5.

In the liquid crystal display panel LCD, a display screen is formed of aplurality of pixels 11 shown in FIG. 2. The array substrate 1 includes aplurality of pixel electrodes 23 arrayed in a matrix form in a displayarea 10 corresponding to the display screen, a plurality of scanningelectrodes 12 extending along the rows of pixel electrodes 23, aplurality of data signal electrodes 13 extending along the columns ofpixel electrodes 23 and a plurality of thin-film transistors (TFTs) 17disposed near the intersections of the scanning electrodes 12 and thedata signal electrodes 13 as pixel switching elements, a plurality ofstorage capacitances 20 at one end connected to the pixel electrodes 23,a scanning circuit 14 connected to the scanning electrodes, and a datasignal circuit 15 connected to the data signal electrodes 13. Eachscanning electrode 12 is connected to the gates of the thin-filmtransistors 17 of a corresponding row, each data signal electrode isconnected to the drains of the thin-film transistors 17 of acorresponding column. Each pixel electrode is connected to the source ofa corresponding thin-film transistor 17. The storage capacitances areconnected at the other end to a common wiring 16. In this case, eachthin-film transistor 17 is formed as an n-channel MOS transistor. Thecounter substrate 2 includes a counter electrode 27 facing the pixelelectrodes 23. Each pixel 11 includes the pixel electrode 23, thecounter electrode 27, and a part of the liquid crystal layer LQ which islocated between the pixel electrode 23 and the counter electrode 27, andforms a liquid crystal capacitance for holding a drive voltagecorresponding to a potential difference between the pixel electrode 23and the counter electrode 27. The storage capacitance 20 is provided inparallel with the liquid crystal capacitance to ensure that the drivevoltage is held stably.

The scanning circuit 14 outputs a scanning signal sequentially suppliedto the scanning electrodes 12 in a direction 7 shown in FIGS. 1 and 2for each horizontal scanning period. The data signal circuit 15 convertsa video signal for one row to pixel voltages while each of the scanningelectrodes 12 is driven by the scanning signal, and outputs the pixelvoltages to the data signal electrodes 13. These pixel voltages areapplied to the pixel electrodes 23 of the row via the thin-filmtransistors 17 for one row which are turned on by the scanning signal.The pixel voltage is a voltage applied to the pixel electrode 23 withrespect to a common voltage applied to the counter electrode 23 andserving as a reference. The pixel voltage is inverted in polarityagainst the common voltage for each frame period and for each horizontalscanning period, for example. The transmittance of each pixel 11 iscontrolled by the drive voltage between the pixel electrode 23 and thecounter electrode 27.

As shown in FIG. 3A, in the array substrate 1, the pixel electrodes 23are formed on a transparent insulating substrate 22 such as a glasssubstrate, and covered with an alignment film 24. In the countersubstrate 2, the counter electrode 27 is formed on a color filter layer26 covering a transparent insulating substrate 25 such as a glasssubstrate, and covered with an alignment film 28. The array substrate 1and the counter substrate 2 are bonded such that the alignment films 24and 28 are set to be in contact with the liquid crystal layer LQ. Theliquid crystal display panel LCD further includes a pair of retardationfilms 30 for optical compensation, a pair of polarizing plates 31, and abacklight 32 serving as a light source. The pair of retardation films 30are attached to the transparent insulating substrates 22 and 25 on thesides opposite to the liquid crystal layer LQ, and the pair ofpolarizing plates 31 are attached to the pair of retardation films 30.The backlight 32 as the light source is disposed next to the polarizingplate 31 on the array substrate 1 side.

In the liquid crystal display panel LCD, the alignment film 24 and thealignment film 28 are subjected to rubbing treatment in directionsparallel to each other. In FIG. 1, RD1 denotes the rubbing direction ofthe alignment film 24, and RD2 denotes the rubbing direction of thealignment film 28. This allows the liquid crystal molecules in theliquid crystal layer LQ to be the splay alignment shown in FIG. 3A atthe initial state. At this initial state, some of the crystal moleculesin the liquid crystal layer LQ in the vicinity of surfaces of thealignment films 24 and 28 are held at high pre-tilt angles (5° to 12°)with respect to the surfaces of the alignment films 24 and 28.

The above-mentioned rubbing treatment is carried out as shown in FIG. 4.In this rubbing treatment, the array substrate 1 and the countersubstrate 2 are prepared in the same manner as other modes, wherein thetwo alignment films 24 and 28 are deposited by coating the arraysubstrate 22 and the counter substrate 25 with an alignment filmmaterial. Each of the array substrate 1 and the counter substrate 2 isplaced on a stage unit such that the long sides are set parallel to therotation axis of a rubbing roller 33, and moved together with the stageunit in the direction of an arrow X across the rubbing roller 33. On therubbing roller 33, a rubbing cloth is wound. This rubbing cloth is setin contact with the alignment film 24 or 28 and rotates along with therubbing roller 33. Thus, each of the array substrate 1 and the countersubstrate 2 is rubbed in the rotational direction of the rubbing roller33, that is, the direction of an arrow Y perpendicular to the rotationaxis of the rubbing roller 33.

In the liquid crystal display panel LCD, upon supply of power, thescanning circuit 14 and the data signal circuit 15 are configured toperform an initialization process for applying a transfer voltagedifferent from the display voltage to all the pixels 11 as the drivevoltage to transfer the alignment state of the liquid crystal moleculesfrom the splay alignment to the bend alignment shown in FIGS. 3B and 3C.FIG. 3B shows the bend alignment obtained upon application of a whitedisplay voltage, and FIG. 3C shows the bend alignment obtained uponapplication of a black display voltage. The white display voltage in theblack insertion driving is set to zero or a small value close to zero,and allows the pixel 11 to be in a high luminous state for transmittinglight from the backlight 32 at the highest transmittance. If the blackinsertion driving is not employed, the white display voltage should beset to a small value (in a range, for example, of 1 to 2 V) at which thebend alignment of the liquid crystal molecules would not beinverse-transferred to the splay alignment. In contrast, the blackdisplay voltage is a value greater than the white display voltage, andallows the pixel 11 to be in a low luminous state for transmitting lightfrom the backlight 32 at the lowest transmittance. The luminance of eachpixel 11 varies in the range between the black display level and thewhite display level in order to display an image.

Further, the scanning circuit 14 and the data signal circuit 15 areconfigured to perform the black insertion driving in the displayoperation after the initialization process. In this black insertiondriving, an inverse-transfer preventing voltage identical to the blackdisplay voltage is sequentially applied to the rows of pixels 11 (theliquid crystal layer LQ) in the scanning direction 7 at a predeterminedblack insertion ratio for each frame period.

In manufacture of the liquid crystal display panel, an aging process isperformed after the above-mentioned forming step in which the arraysubstrate 1, the counter substrate 2, the liquid crystal layer LQ, thepair of alignment films 24 and 28, for example, are formed as onedisplay panel. In the aging process, a black display voltage and a whitedisplay voltage are alternately applied to the liquid crystal layer LQfor each frame period to obtain a flow of the liquid crystal moleculesin the rubbing directions of the alignment films. This aging process iscontinued for a predetermined period required for moving impurity ionscontained in the liquid crystal layer LQ outside the display area 10.The black display voltage and black insertion ratio for the agingprocess are determined independently from the black display voltage andblack insertion ratio of the black insertion driving for preventing thealignment state of the liquid crystal molecules from beinginverse-transferred from the bend alignment to the splay alignment. As aresult of the aging process, the concentration of the impurity ionscontained in that part of the liquid crystal layer LQ which is locatedinside the display area 10 is set to a low value that falls within arange of ½ to ⅕ of the concentration of the impurity ions contained inthat part of the liquid crystal layer LQ which is located outside thedisplay area 10.

In the case where the rubbing directions RD1 and RD2 coincide with thescanning direction 7 from the uppermost side to the lowermost side ofthe screen as shown in FIG. 1, the impurity ions are drifted downward inthe pixels 11 of one row by the drive voltage which is periodicallychanged between the white display voltage and the black display voltage,and such downward drift also occurs in the pixels 11 of the subsequentrows sequentially scanned. Thus, the impurity ions are continuouslymoved towards the lower end of the screen as indicated by an arrow 8,and ultimately swept out from the display area 10. Accordingly, byperforming the aging process, it is possible to suppress non-uniformityin display caused by the impurity ions unevenly concentrated in thedisplay area 10 during the black insertion driving for displaying animage on the liquid crystal display panel.

Once the rubbing directions RD1 and RD2 have been determined to coincidewith the scanning direction 7 as described above, the movement ofimpurity ions can efficiently be carried out in the aging process. Evenif the directions RD1 and RD2 are not opposite to the scanning direction7, it is possible to ensure sweeping of the impurity ions byappropriately setting the conditions for the aging process. For example,the black display voltage for the aging process has been set to 6 V,which is slightly greater than the black display voltage (=5 V) for theblack insertion driving. When this voltage for the aging process fallswithin a range, for example, of 5 to 6 V, an improved effect can beobtained in the aging process.

It was found that the efficiency of sweeping the impurity ions wasimproved by an increase in the black insertion ratio for the agingprocess, that is, the ratio of the application period of the blackdisplay voltage to the total application period (1 frame period) of theblack and white display voltages. However, an optimum value ranging from50 to 80% exists in the black insertion ratio for the aging process. Inthe embodiment, the black insertion ratio of 70% is employed in theaging process and a favorable effect was obtained.

It was also found that the higher the aging process temperature, themore the impurity ions were swept out. In the embodiment, the agingprocess temperature was set to 60° C. and a favorable effect wasobtained. However, when the process temperature is high exceeding NItemperature, the effect was declined. The polarization plate may bedamaged by an increase in the aging process temperature. Hence, theupper limit of the aging process temperature is 70° C. It was found thatthe effect by heating was somehow noticeable at 40° C. or higher.

The longer the aging process time, the more the effect will beincreased. It was found that one hour or longer was enough to obtain theeffect of sweeping the impurity ions. The effect was saturated when theaging process continued for twelve hours. Therefore, the length of theaging process time is preferably between one hour and twelve hours. Whena relationship between the sweeping effect of the impurity ions and theaging process time is considered from a view point of the productionefficiency, the length of the aging process time is preferably three tofive hours. In the embodiment, the aging process time was set to threehours and a favorable effect was obtained.

In addition, it is preferable that a dummy electrode is provided in anexternal area located next to the upper or lower side of the displayarea 10 which corresponds to an end in the rubbing direction, and awhite display voltage is always applied to the dummy electrode. In thiscase, it is possible to obtain an effect in which the impurity ions aremoved ultimately to a position apart from the side of the display area10.

Although the embodiment of the present invention is intended for theelimination of non-uniformity in display that is observed as burn-inportions appearing near the boundary between the black and white displayregions, the present invention is not limited to the embodiment andapplicable to other types of non-uniformity in display. For example,there is a case where non-uniformity in display is observed at alocation which is inner by about 2 mm in the black display regionbecause of thermal diffusion. Also, in the embodiment, the flow in therubbing direction is a primary cause for drift of the impurity ions inthe description. However, there is a case where the impurity ions aredrifted by the flow in an opposite direction. In other words, it isdetermined which flow the drift is caused depending on the mobility ofions. In any case, the non-uniformity in display can be suppressed ifthe impurity ions are successfully swept away by means of theaforementioned aging process.

In the aforementioned embodiment, the scanning circuit 14 and the datasignal circuit 15 are composed of the thin-film transistors formed onthe array substrate 1 of the liquid crystal display panel LCD. Thesecircuits 14 and 15 may be replaced by integrated circuits (ICs) that areprovided externally to the liquid crystal display panel LCD.

Further, the present invention is applicable to OCB mode liquid crystaldisplay panels in which no black insertion driving is performed. In thiscase, the bend alignment is easily inverse-transferred to the splayalignment if the white display voltage of 0 V is used for the agingprocess. To prevent the inverse-transfer, it is preferable that thewhite display voltage for the aging process is set to a small levelwhich falls within a range of 1 to 2 V.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A method for manufacturing an OCB mode liquid crystal display panel,which comprises: forming a pair of electrode substrates, a liquidcrystal layer which is held between said pair of electrode substratesand has liquid crystal molecules whose alignment state is controlledwithin a display area by a drive voltage applied from said electrodesubstrates, and a pair of alignment films which are disposed on saidelectrode substrates in contact with said liquid crystal layer and whoserubbing directions are set parallel to each other, as one display panel;and performing an aging process for obtaining a flow of the liquidcrystal molecules in the rubbing directions of the alignment films byapplying to said liquid crystal layer a drive voltage that periodicallychanges in a black insertion ratio determined independently from that ofblack insertion driving for preventing the alignment state of the liquidcrystal molecules from being inverse-transferred from a bend alignmentto a splay alignment, said aging process being continued for apredetermined period required for moving impurity ions contained in saidliquid crystal layer outside said display area, wherein substantiallyblack and substantially white display voltages are alternately appliedfor each frame period as the voltage for said aging process, and saidpredetermined period is within a range of 1 to 12 hours for which saidaging process is continued.
 2. The method according to claim 1, whereinthe black insertion ratio in said aging process exceeds the blackinsertion ratio for the black insertion driving.
 3. The method accordingto claim 2, wherein the black insertion ratio in said aging process is50% or more.
 4. The method according to claim 3, wherein the blackinsertion ratio in said aging process falls within a range of 50 to 80%.5. The method according to claim 1, wherein the black display voltage insaid aging process is greater than the black display voltage for theblack insertion driving.
 6. The method according to claim 1, whereinsaid aging process is performed at an aging process temperatureexceeding 40° C.
 7. The method according to claim 1, wherein saidpredetermined period falls within a range of 3 to 5 hours for which saidaging process is continued.
 8. An OCB mode liquid crystal display panel,comprising: a pair of electrode substrates; a liquid crystal layer whichis held between said pair of electrode substrates and has liquid crystalmolecules whose alignment state is controlled within a display area by adrive voltage applied from said electrode substrates; and a pair ofalignment films which are disposed on the electrode substrates incontact with the liquid crystal layer and whose rubbing directions areset parallel to each other; wherein the concentration of the impurityions contained in that part of said liquid crystal layer which islocated inside said display area is lower than the concentration of theimpurity ions contained in that part of said liquid crystal layer whichis located outside said display area, and wherein a dummy electrode isfurther provided in an external area located next to a side of saiddisplay area corresponding to the end of the rubbing direction and thatreceives a voltage always applied thereto, said liquid crystal layer issurrounded by a sealing layer between said electrode substrates, and aliquid crystal inlet is provided in a part of said sealing resin layeron a side of the end of the rubbing direction and sealed by a sealingmember.